How Did Philip the Great Conquer the Persian Empire in 20 Years?

Alexander magnus was the conqueror and destroyer of the persian empire. He inherited the macedonian kingdom that's north of greece when he was 20 turning 21. His father had remarried a young girl and he and she had had a baby and he was going to be out so he was i don't know whether he was involved in the assassination of philip the second but everybody's would like to know how did this little backward place in in northern greece do it in 20 years.